1. Field of the Invention
This invention relates generally to hard disk drives for use in magnetic recording, and in particular, relates to techniques for resisting remanent magnetization in head components.
2. Background Information
Hard disk drives (HDDs) manage a large capacity of information being written to and retrieved from the media of disk platters controlled by the drive. A hard disk drive includes I/O interfaces, read and write head components, encoding and decoding components and servo mechanisms that appropriately position the respective heads over the recording media. These components are controlled by integrated circuits which perform signal processing, encoding and decoding and control the operation of a given head as it writes data to and retrieves data from a disk. In particular, a hard disk drive contains an HDD controller circuit which is coupled to the I/O interface and receives input data and transmits data that has been read from the drive. The hard disk drive also includes a read/write channel integrated circuit, which is referred to herein as the “read channel.” During operation, the read channel receives a digital data stream and encodes the data by a variety of modulation processes into an encoded data sequence. The read channel causes the encoded data to modulate voltage reversals such that, for example, a voltage reversal occurs corresponding to the center of each data cell where a “1” occurs. If the data cell contains a “0” no voltage reversal occurs. The read channel uses digital signal processing techniques in addition to data encoding schemes and digital filtering to optimize this process. The read channel also decodes servo-information used for positioning drive heads during seeking and tracking operations.
The alternating voltage pattern corresponding to the encoded input data stream is send to a preamplifier integrated circuit, sometimes referred to as the “preamplifier.” The preamplifier amplifies the signal from the read channel and produces an alternating current sequence corresponding to the timing of the input voltage changes. This current signal is then applied to the drive head to write the data as a series of magnetic transitions on the disk media in a write operation. The preamplifier circuit, in a read operation, amplifies analog signals that correspond with data read from the disk before such signals are sent to the read channel for digitization.
In a write operation, a write gate signal is asserted by either the read channel or an HDD controller, and the signal is sent to the preamplifier. The preamplifier responds with its transducer current amplitude being either on or off in response to this write gate signal. Thus, the write gate signal, when asserted, causes the preamplifier to turn the transducer current amplitude on. As described in further detail herein, the preamplifier reverses transducer current polarity based upon write data sent to it from the read channel. Specifically, the write transducer contains magnetic poles and one or more drive coils that convert the electrical signals to magnetic signals, which in turn cause the magnetic domains in the disk media to transition (change directions) or remain unchanged in direction.
The hard disk drive follows a procedure to properly end a write operation. The first event of this sequence is the de-assertion of the channel write gate signal, which in turn causes the read channel to flush the data symbols that were input to the channel up to this point. Thereafter, the channel begins output of a “postamble” pattern. This postamble pattern is a set of transitions used to provide an end condition needed to reliably identify the last data symbol written upon read back. After sufficient postamble has been output, the preamplifier write gate signal is de-asserted, which causes the preamp to turn off write current. However, this turn off is not instantaneous, and in practice, the write current output of the preamplifier takes some time to decay.
This final write current polarity and amplitude can result in remanent magnetization at the poles of the write transducer or in the yoke of the drive head. It takes some time for this remanent magnetization to relax. More specifically, after several nanoseconds it has been found that only 60% of the remanent magnetization may have relaxed, which means that up to 40% of the remanent magnetization continues to be emanated from the write transducer of the head. Thus, spurious data may be written to the disk by this write transducer remanent magnetization, or valid data may be erased from the media by continuing the magnetic field from the write transducer into the next track or sector of the disk. If the magnetic remanence continues unchecked over a greater time period, it can eventually lead to pole tip lockup or a lockup of another part of the head, such as the yoke, which is unacceptable.
In order to address the problem of remanent magnetization after a write operation, it has been suggested in the prior art to provide the preamplifier circuit with an oscillator that generates an AC signal, to perform an AC de-gaussing operation. However, this requires an oscillator, which is not otherwise necessary, to be included in the preamplifier thereby resulting in additional costs and complexities in manufacturing the preamplifiers. Such a redesign of preamplifiers not only increases the overall cost of manufacturing and design of the hard disk drive, but may also result in performance degradation. Specifically, the types of oscillators that are suggested for the preamplifiers can have poor frequency tolerance that can result in format efficiency loss in lower frequency conditions. Furthermore, many drives use preamps of different types and from different vendors, such that there are part-to-part variations among manufacturers. A preamplifier based de-gaussing solution would require a consistent circuit across a plurality of preamplifiers and a plurality of vendors, and this could become infeasible due to the part-to-part variations that already exist among the preamplifiers from various vendors.
Further disadvantages faced in providing an oscillator in the preamplifier include the fact that the preamplifier chip is usually highly temperature sensitive and voltage sensitive. Thus, the exact output of an oscillator signal produced in a preamplifier cannot always be predicted due to this variation in operating characteristics.
Some other solutions that have been proposed in the prior art include laminating the pole tips in order to reduce the effect of magnetic remanence as the write current decays. However, pole tip lock up can occur even with such lamination, if the remanent field strength exceeds the coercivity of the media. Furthermore, laminating the pole tips will not address the problem of yoke lockup due to remanent magnetization after a write operation.
Other factors can increase the adverse effects of remanent magnetization. For example, the risk of unwanted erasure due to pole tip lock up is further enhanced in zones of the disk or tracks which are exposed over and over. In fact, there can be a 25% amplitude loss in the magnetics contained within overexposed zones, thus making them more susceptible to erasure. Remanent magnetization can also lead to unwanted domain walls which can contribute to the lock up. Areas of the media that may have been subjected to a DC erase process also contribute to the potential for lock up, particularly in a perpendicular magnetic recording system.
It is thus an object of the invention to provide a de-gaussing solution for an HDD, which substantially eliminates drive head component lockup due to remanent magnetization in HDDs that does not require a redesign of the preamplifier or other components and does not add further costs to manufacture of the HDD.